1. Field of the Invention
This invention, generally relates to electrochemical capacitors and, more particularly, to a supercapacitor made from a hexacyanometallate cathode, activated carbon anode, and aqueous electrolyte.
2. Description of the Related Art
A supercapacitor, or electrochemical capacitor (EC), constitutes one type of device for electrochemical energy storage and conversion. The electrochemical capacitor consists of two electrodes separated by an electrolyte-soaked separator by which the two electrodes are electrically isolated. Based upon the electrode type and the energy storage mechanism, the supercapacitor may be classified into one of the two following categories: (1) an electric double layer capacitor (EDLC), in which the energy is stored at the interface between the electrode and electrolyte through electrostatic charge accumulation, or (2) a pseudo-capacitor (or ultra-capacitor), whereby one electrode undergoes faradic reactions while the other electrode maintains the electrostatic charge accumulation.
In comparison to batteries and fuel cells, supercapacitors have the highest power density (watt per kilogram) due to the rapid movement of ions between the electrode and electrolyte. However, at the same time, the energy storage mechanism restricts its energy density (watt-hours per kilogram). Charge adsorption on the electrode surface dominates the energy storage in EDLCs. The performance of electrode materials can be evaluated by capacitance (F/g, Farad/gram), which can be defined as the charge accumulation with the voltage change per gram of electrode material: C=(ΔQ/ΔV)/m where Q is the accumulated charge, V is the voltage of the electrode materials, and in is the mass of the active materials of the electrode. The state-of-the-art commercial activated carbon materials have surface areas of 1000-3500 m2/g and capacitances of ˜200 F/g [L. L. Zhang, X. S. Zhao, “Carbon-based materials as supercapacitor electrodes”, Chem. Soc. Rev., 38 (2009)2520-2531]. Physical adsorption of the electrostatic charge restricts the capacitance from further increases. Therefore, the introduction of faradic reactions to supercapacitors results in the so-called pseudo-/ultra-capacitors.
In general, pseudo-capacitors demonstrate much larger capacitances than EDLC because Faradic reactions can store charges both on the surface and in the bulk of the electrode materials. Ruthenium oxide (RuO2), for example, exhibits a high capacitance of 720 F/g [J. P. Zheng, P. J. Cygan, T. R. Jaw, “Hydrous ruthenium oxide as an electrode material for electrochemical capacitors”, J. Electrochem. Soc., 142 (1995) 2699-2703] based upon the faradic reaction of RuOx(OH)y+zH++ze−←→RuOx−z(OH)y+z where the redox couple, Ru3+/4+, is reversible during the dis/charge process. Except for the appropriate redox couples, a robust material for the pseudo-capacitor electrode must demonstrate fast transport of charges and electrons in its structure(s), in order to ensure a high power density. Although Faradic reactions increase the capacitance of electrode materials significantly, both the transfer of charges and migration of ions result in a change of volume, which deteriorates their structure during cycling. For that reason, pseudo-/ultra-capacitors exhibit a shorter cycling life than EDLCs. Thus, more stable materials are actively being developing for this class of capacitor.
FIG. 1 depicts the crystal structure of a metal hexacyanometallate (prior art). Prussian blue analogues belong to a class of mixed valence compounds called transition metal hexacyanometallates. The hexacyanometallates have a general formula AmM1xM2y(CN)6, where M1 and M2 are transition metals. In many cases, the transition metal hexacyanometallates may contain a variety of ions (A=Co+, Na+, K+, NH4+, Co2+, Cu2+, etc.) and various amounts of water in the crystal structure. As is shown in the figure, the crystal structure of metal hexacyanometallates has an open framework which can facilitate fast and reversible intercalation processes for alkali and alkaline ions (Am). The number of alkali or alkaline ions in the large cages of this crystallographically porous framework may vary from m=0 to m=2 depending on the valence of M1 and M2.
Twenty years ago, Widmann, et al. demonstrated that K+-ions reversibly insert/deinsert into/from the copper, nickel, and iron hexacyanoferrates/hexacyanocobaltates Prussian blue analogues, KNiFe(CN)6, KCuFe(CN)6, and KFeFe(CN)6) in aqueous solution [A. Widmann, H. Kahlert, I. Petrovic-Prelevic, H. Wulff, Jr Yakhmi, N. Bagkar, F. Scholz, “Structure, insertion electrochemistry, and magnetic properties of a new type of substitutional solid solution of copper, nickel and iron hexacyanoferrates/hexacyanocohaltates”, Inorg. Chem., 41 (2002) 5706-5715].
Eftekhari [A. Eftekhari, “Potassium secondary cell based on Prussian blue cathode”, J. Power Sources, 126 (2004) 221-228] assembled an iron hexacyanoferrate (Prussian blue)/potassium battery with an organic electrolyte [1M KBF4 in ethylene carbonate/ethylmethyl carbonate (3:7 by wt.)]. The results proved that Prussian blue was a good electrode material for the potassium-ion battery with a reversible capacity of ca. 75 mAh/g.
Dr. Goodenough's group [Y. Lu, L. Wang, J. Cheng, J. B. Goodenough, “Prussian blue: a new framework of electrode materials for sodium battery”, Chem. Commun. 52 (2012)6544-6546] investigated a series of Prussian blue analogues in a sodium battery with organic electrolyte, and found that KFe(II)Fe(III)(CN)6 demonstrated the highest capacity of ca. 95 mAh/g, while KMnFe(CN)6, KNiFe(CN)6, KCuFe(CN)6, and KCoFe(CN)6 demonstrated a capacity of 50˜70 mAh/g, in the first 30 cycles, the capacity retention of KFeFe(CN)6 was higher than 97%.
Very recently, Cui's group studied the Na+/K+ ion intercalation of copper (KCuFe(CN)6) and nickel hexacyanoferrates (KNiFe(CN)6) in aqueous solution. Their results demonstrated the rapid movement of Li+, Na+, K+, and NH4+-ions in the Prussian blue analogues, as well as long cycling life for the electrode materials [C. D. Wessells, R. A. Huggins, Y. Cui, “Copper hexacyanoferrate battery electrodes with long cycle life and high power”, Nature Communication, 2 (2011) 550; C. D. Wessells, S. V. Peddada, R. A. Huggins, Y. Cui, “Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries”, Nano Lett., 11 (2011) 5421-5425; C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, Y. Cui, “The effect of insertion species on nanostructured open framework hexacyanoferrate battery electrode”, J. Electrochem. Soc., 159 (2012) A98-A103].
The fast ion movement makes it possible for the Prussian blue analogues to perform effectively as the electrode in supercapacitors. It is worth noting that iron ions at the M2 site in the above-mentioned materials remain at a high oxidation state (Fe3+) and the materials have to accept more intercalated ions to reduce the iron-ions (Fe2+), which requires the sources for the intercalated ions to couple the Prussian blue electrode. On the other hand, the Mn2+, Ni2+, Cu2+ and Fe2+ at the M1 site need a high voltage to deinsert the A-ion and to oxidize the M1 to the 3+ state. The voltage is so high that H2O decomposes in an aqueous electrolyte prior to oxidizing the M1. Therefore, only the Fe at the M2 site can perform the redox cycle (Fe2+/Fe3+) in an aqueous electrolyte for this supercapacitor application. If the Prussian blue analogues for a supercapacitor application have a high oxidation state (Fe3+) at the M2 site, the anode must source the supply of the intercalated ions. Cui's group, for instance, used a large, partially charged Prussian blue electrode as the counter electrode to provide a Na+ or K+ source. Therefore, it is impossible to use these materials in supercapacitors because the other electrode must be activated carbon, which cannot function as an ion-source for the Prussian blue analogues.
It would be advantageous if a supercapacitor could be fabricated using a Prussian blue cathode and an activated carbon anode.
For greater safety, it would be advantageous if the above-mentioned supercapacitor could be fabricated with an aqueous electrolyte.